Learn English – Is It Ok To Write “100%” In A Formal Text

You probably don't want to do it at all. Noninteger numbers are best written in their decimal form (1.5, 5.0). If they can be expressed as simple fractions, you can do that instead: “one and a half” for 1.5.

If you absolutely want to write it out, then you'll have to do it the way they are spelt: “one point five”, “five point zero”. You can use oh instead of zero (“five point oh”, “oh point five”) to reflect an informal spoken style.

I wonder: if you must complete the form using words, is it necessary to include the fractional part? One could say:

• I own approximately twelve hundred and ten shares.

or,

• I have just over one thousand, two hundred and ten units.

If you must be absolutely precise, though, you might use:

• One thousand, two hundred and ten units, plus eight hundred and two thousandths of a unit.

My goodness, though! After you write such a monstrosity, do what Barrie suggested, and put the number in parentheses.